Page 130 - Geochemical Remote Sensing of The Sub-Surface

Spontaneous potentials and electrochemical cells 107

by oxidation can it pass on charge. Second, the cathode, like the anode, must be at least
as reducing as Fe 2+, otherwise the sulphide would be locally oxidised in its upper end
and no SP phenomenon would result.
The pyrrhotite-to-pyrite reaction is just one example of a process that could generate
such a cell. Another is pentlandite-to-violarite (Thornber, 1975a) and there are many
other possibilities. The differentiation of a sulphide into oxidised and reduced phases in
the upper and lower portions respectively is, in itself, a product of the operation of this
type of cell. If subjected to long term oxidation, a deep pyrrhotite body will slowly
oxidise to pyrite as the pyrrhotite/pyrite boundary progresses downward. These cells
therefore require a deep weathering profile where: (1) there has been abundant time for
the oxidised/reduced sulphide front to migrate downward; and (2) most species in the
bedrock/groundwater environment surrounding the conductor that are more reducing
than the reduced species (e.g., pyrrhotite) have already been consumed. As such, one
would expect the development of cells such as these in old geological terrain but not in
younger terrain such as continentally-glaciated areas. However the reactive groundwater
and reactive conductor models may be end members and probably both operate to
varying degrees wherever conductor-based cells occur.

Cells in the absence of electronic conductors

Virtually all discussion in mineral exploration regarding SP cells and associated
electrochemical phenomena assumes the presence of an electronic conductor. There has
been little discussion of voltaic cells that involve no electronic conduction, but these
cells undoubtedly exist. The nervous systems and muscles of organisms use the transfer
of purely ionic current with no electronic conduction. Spontaneous potentials in the
absence of electronic conductors have long been recognised in the petroleum industry
and result from salinity and redox differences between strata. The presence of
spontaneous potentials has also been noted in relatively thick overburden overlying
mineralisation in the absence of an overburden conductor of electrons (Burr, 1982).
Since electrons cannot move freely in an electrolyte solution, many of these cases must
involve electrochemical cells of sorts in which current is transferred exclusively in the
form of ions.
Two types of SP cells have been postulated to develop in media with presumably
homogenous resistivity. These are SP cells over bedrock mineralisation and deep
hydrocarbon-based cells in bedrock. These cells are not centred on zones of elevated
electrical conductivity but rather on zones of elevated SP (voltage) gradient.
In both the reactive groundwater and reactive conductor models, the impetus for
electronic current flow in mineralisation comes from the redox differential between the
oxidised groundwater environment surrounding the upper part of the conductor and
reducing agents in contact with its lower part. The upward movement of electrons
consumes oxidising agents in basal overburden and results in the development of a